Electron discharge device utilizing negative transconductance



Dec. 31, 1940. E. w. HEROLD 2,226,551

ELECTRON DISCHARGE DEVICE UTILIZING NEGATIVE TRANSCONDUCTANCE Filed Oct. 22. 1958 k z our/ 01 WC'I'w/PK .h M 5 9 m m R R. Y. E. u mm m H5? 9 Ha mm w h wm a 2 WW N Z Z I A l m k my; .w. W0 -A MAME A Mz Z q M I c A W 5 ll 25. I 2 K. .El 7 5 4 FM M c A 6/ M mam Mam G w N 0M R H MM 7 1 ea 6 m mm W MN fl 0r k Mb .MJ 4 z K r Q! L f MW H Or I! w 11111 1 w c w Patented Dec. 31, 1940 ELECTRON DISCHARGE DEVICE UTILIZING NEGATIVE TRANSCONDUCTANCE Edward W. Herold, Verona, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 22, 1938, Serial No. 236,381

14 Claims.

This invention relates generally to electron discharge device circuits, and more'particularly to electron charge device oscillation generators utilizing the negative transconductance phenomena.

In the conventional type of evacuated electron discharge device or vacuum tube, a rising potential on the control electrode causes an increase in the electron current to the output electrode. Such a device is said to possess positive transconductance. In another type of evacuated electron discharge device, a rising potential on the control electrode causes a decrease in the electron current to the output electrode. This last type of device is said to possess negative transconductance. Both of these types of devices may be used either as amplifiers or oscillation generators, it being understood that electron discharge device oscillators function by virtue of the negative resistance which they produce. In the case of an oscilation generator, the negative transconductance type differs from the positive transconductance type in the relative phase of the feed-back necessary to produce negative resistance, that is, to sustain oscillations. For example, in the more conventional device, it is necessary to reverse the phase of the current fed back from the output electrode to the control electrode, as in 39 stanced by means of a tickler coil or some reactive circuit. With negative transconductance electron discharge devices, however, oscillations may be sustained by a direct coupling between the output and control electrodes. For a more com- 5 plete discussion of negative transconductance devices and their application to the production of negative resistance, reference may be had to my article entitled Negative Resist-ances and Devices for Obtaining It, published in the Proceedings of the I. R. E. for October, 1935,pages The primary object of the present invention is to improve the performance of a negative transconductance oscillation generator.

45 This object is achieved by the use of feedback from an auxiliary anode electrode to an auxiliary control el-ectrodathe circuit being so arranged that said auxiliary control electrode draws little or no current. This feed-back, ac-

50 cording to the invention, is separate and apart from any feed-back necessary to produce oscillations. By means of the present invention there is obtained a very substantial increase in the effective negative transconductance. This as increase. in effective negative transconductance may easily exceed tenfold the negative transconductance obtained in known types of circuits which do not utilize the auxiliary feed-back arrangement of the invention. Such an increase,

it is to be distinctly understood, is merely given 6 by way of illustration, and not by way of limitation, since much greater increases may also be obtained.

A better understanding of the invention may be had by referring to the following description, 10 which is accompanied by a drawing wherein:

Figs. 1 and 2 show, in simplified form, two different embodiments of negative transconductance devices in accordance with the invention, used as amplifiers. ments are given to illustrate the principles of the invention as subsequently applied to oscillation generator circuits;

Figs. 3 and 4 show negative transconductance oscillation generator devices, in accordance with 20 the invention, of the voltage-controlled type;

Figs. 5 and 6 show other embodiments of negative transconductance oscillation generator devices, in accordance with the invention, of the current-controlled type; and 25 Fig. 7 shows a preferred form of negative transconductance oscillation generator device, in accordance with the invention, using an improved type of vacuum tube.

Throughout the drawing, corresponding parts are designated by the same reference characters.

Referring to Fig. 1, there is shown an evacuated pentode electron discharge device I (i. e.,

a vacuum tube) having a cathode K, a first grid G1, a second grid G2, a third grid G3, and a plate electrode A. Asuitable inputcircuit 2, 2 is connected between the cathode K and the third grid G3, the latter being used as the main control electrode 'A suitable output circuit 3 is connected between the cathode K and the sec- 40 0nd grid G2, the latter being used as the output electrode. The main control electrode or third grid G3 is maintained at a negative potential by battery 4 (through the input circuit 2, 2),

so as not to draw current, while the output electrode or second grid (32 is maintained at a positive potential by battery 5 (through the output circuit 3, 3). The first grid G1 is maintained at a negative potential by the battery 6, but also has impressed upon it potential variations occurring across impedance Z whose nature will be described later. Control electrode G1 is used for auxiliary control of the electron stream within the tube, emanating from cathode K. The outer plate electrode A is an aux- These amplifier embodiiliary anode electrode which is maintained at a positive potential by battery I. Current variations from this outer electrode A set up pctential variations across the impedance Z which are impressed on the auxiliary control electrode G1.

The manner in which the effective value of the negative transconductance occurs between electrodes G3 and G2 will now be described: When an increase in potential from the input circuit 2, 2 is applied to the main control electrode Ga, there will be a decrease in the current to the output electrode G2. This will be apparent from the fact that a more positive potential on G3 will draw more electrons through this electrode, and consequently diminish the number ofelectrons impinging on electrode G2. Conversely, a decrease in the potential applied to the main control electrode G3 from the input .circuit will retard the electron flow through this last electrode and hence cause an increase in the electron current to the output electrode G2. When the increase of potential from the input circuit is applied on main control G3, the current to the auxiliary anode A increases in correspondence to the decrease in current to the main output electrode G2. This increase in current to the auxiliary electrode A causes the potential drop across the impedance Z which produces a decrease in potential on the first grid or auxiliary control electrode G1, thereby diminishing'the total electron current which is permitted to pass through electrode G1 from the cathode to the remaining electrodes. This diminution of cathode current assists materially in the reduction of current to the main output electrode G2 caused by the aforementioned increase in potential on main control electrode G3, thus augmenting the control action of electrode G3. Similarly, a decrease in potential on main control electrode G3 caused by the decrease of potential in the input circuit will'reduce the electron flow to auxiliary electrode-A, thus reducing the potential drop across impedance Z and increasing the potential on the first grid or auxiliary control electrode G1.

,This increase in potential 1OI1'G1 will increase the electron flow through this electrode from the cathode to the remaining electrodes, thus augmenting the increase in current to the output electrode G It will thus be evident that I progvide a feed-back path from the auxiliary electrode A to the auxiliary control grid G1 which augments or assists the ordinary or conventional negative transconductance produced between electrodes G3 and G2.

The impedance Z may, in its simplest form, consist of a pure resistance. As such it is suitable for the production of potential variations due to current in the auxiliary anode A at comparatively low frequencies, under 100 kilocycles,

-for example. A disadvantage in the use of the simple resistance for impedance Z is that there exists unavoidable capacitances across the same, thus making it unsuitable at the higher frequencies inasmuch as these unavoidable capacitances shunt the resistance and reduce the effectiveness of the feed-back path. Consequently, in the operation of the vacuum tube circuit over a wide band of frequencies extending up to high frequencies of the order of a megacycle and higher, it is preferred that the impedance Z take the form of a suitable network. Such networks may take any one of the following forms: A small inductance in series with a relatively high resistance, an inductance shunted by a capacitance in series with a resistance, or an inductance in series with a resistance with a series connection of inductance and capacitance shunted across the resistance. Still other networks may be devised for the impedance Z which consist of filter networks with dead-end terminations capable of giving a substantially constant resistance over a wide band of frequencies. Where a single frequency or a narrow band of frequencies is involved, impedance Z may take the form of a parallel resonant circuit tuned to the frequency or to the center of the band of frequencies involved. In circuits whose operating frequency is to remain very constant, a piezo-electric crystal may be used as a part of impedance Z. The selection of a suitable impedance for the specific requirements is easilymade and will be obvious to anyone skilled in the art.

One disadvantage of the circuit of Fig. 1 is that batteries 6 and l are floating or at a different potential from batteries 4 and 5, the latter of which are directly connected to the cathode K. The circuit of Figp2, which also shows an amplifier arrangement of a type similar to Fig. 1, overcomes this disadvantage by connecting all batteries directly to the cathode, thus, in effect, minimizing the unavoidable capacitance previously existing across impedance Z of Fig. 1. In the case of Fig. 2, the potential variations across impedance Z are impressed on the grid G1 through the capacitance C1. The grid bias on G1 is provided through the comparatively high resistance R1. The circuit of Fig. 1 is effective at all frequencies, including zero (i. e., the direct current case). The circuit of Fig. 2 is effective only at frequencies greater than zero because only alternating current variations are transmitted through the capacitance C1. Outside of these differences, the circuit arrangements of Figs. 1 and 2 are the same and the operation is the same.

Fig. 3 shows how a negative transconductance device circuit of the type shown in Fig. 1 can be used as an oscillation generator, in accordance with the invention. In this case, the main control electrode G3 is directly connected to the output electrode G2 through a battery 8, arranged to polarize the electrode G3 at a negative potential with respect to the cathode K. This negative potential on G3 prevents main control electrode G3 from drawing appreciable amounts of current. Because of the direct connection between the output electrode G2 and the main electrode G3, potential variations across points 9 and 19 are impressed directly on control grid G3. An increase in'potential on point 9 will cause an increase in potential on G3 and a decrease in current to G2. Similarly, a decrease in potential on point 9 will cause a decrease in potential on the electrode G3 and an increase in electron current to G2. The result, looking into the electron discharge device circuit from points 9 and I0, is that the device I will produce a negative resistance across these points. Thus, by the connection of a parallel tuned circuit l I across points 9 and I0, sustained oscillations may be generated. It will be evident from an inspection of Fig. 3 that there are present two feed-back circuits, one of which is the direct coupling or main feedback path between grid electrodes G2 and G3 through battery 8, and the other of which is the auxiliary feed-back circuit from the electrode A to the auxiliary control grid G1 through impedance Z. It should be noted that the auxiliary feed-back path between the electrodes A and G1 is separate and apart from the main feedback path between electrodes G2 and-Gs and plays no part in the generation of oscillations, serving only to augment the negative transconductance between grids G2 and G3. The impedance Z may, of course, take any one of the forms previously discussed in connection with Fig. 1. However, if impedance Z is a tuned circuit tuned to the operating frequency, it augments the negative transconductance only at this frequency, and thus adds to the frequency stability of the complete circuit. In effect, the circuit of Fig. 3 produces a voltage-controlled negative resistance.

The oscillation generator of Fig. 3 possesses the same-disadvantage which exists in Fig. 1, namely that the batteries 6 and I, as well as battery 8, are floating or at a different potential than battery 5 with respect to the cathode. Just as the amplifier circuit of Fig. 2 is designed to overcome this disadvantage of Fig. 1, so the oscillator generator of Fig. 4 is designed to overcome the same disadvantage of Fig. 3. Fig. 4 is also a voltage-controlled negative resistance oscillation generator which bears the same relation to Fig. 2 as the oscillation generator of Fig. 3 bears to Fig. 1. The main feed-back path of Fig. 4

consists of a capacitive coupling C2 bridged across the main output electrode G2 and the main control electrode G3. The main grid electrode G3 obtains its bias through a comparatively high resistance R2. These diiferences between Fig. 4 and Fig. 2 serve to change the amplifier circuit of Fig. 2 into an oscillation generator, the circuit otherwise remaining the same. The oscillator of Fig. 4 functions to produce negative resistance in a similar manner as that of Fig. 3, the auxiliary feed-back path from auxiliary electrode A to auxiliary control grid G1 functioning in the same manner as in Fig. 3 to augment the negative transconductance between grids G3 and G2.

It should be noted that the oscillations generated by the circuit of Figs. 3 and 4 are determined by the resonant frequency of parallel resonant circuit II. In the case of Fig. 4, the oscillator may be further modified to take the form of a relaxation oscillator merely by replacing the parallel tuned circuit II by a suitable resistor, which may or may not be shunted by a capacitance. In this case, the frequency of oscillation would be determined by the time constants of circuits C2, R2 and C1, R1, and the time constant of the resistor and capacitor which replaced the parallel tuned circuit II. Oscillations may also be generated by other circuit arrangements, as for example with a piezo-electric crystal connected in place of R2 or C2 of Fig. 4.

Fig. 5 shows the principles of the invention applied to an oscillation generator of the current-controlled negative resistance type. The auxiliary feed-back of Fig. 5 is similar to the auxiliary feed-back in Fig. 1, but the main feedback which causes the current-controlled negative resistance is obtained by applying to G3 the voltage drop across resistor R3 which is in series with the series tuned output circuit I l and output electrode G2. The bias provided by battery 12 serves to keep the potential of G3 negative with respect to the cathode. The output circuit l l of this figure is a series tuned circuit instead of the parallel tuned output circuits of Figs. 3 and 4, since the main feed-back applied to grid G3 is now proportional to the current in the output circuit ll, whereas in Fig. 3 and Fig. 4 it was proportional to the voltage across the output circuit II. In order to provide a direct currentpath to the main anode, the circuit element C, which may be a choke coil or resistance,

is employed. It will be obvious, of course, that the series tuned circuit II" is arranged to allow maximum current and therefore maximum feedback at resonance, whereas in Figs. 3 and 4 the output circuits H are arranged to produce maximum voltage and thus maximum feed-back at resonance. p

The current-controlled negative resistance oscillation generator circuit of Fig. 5 also possesses the same disadvantage as the circuits of Fig. 1 and Fig. 3 of having different batteries at different potentials relative to the cathode. This disadvantage oii the current controlled negative resistance oscillator circuit of Fig. 5 can be overcome by means of the oscillator circuit of Fig. 6 which also shows a current-controlled negative resistance oscillator circuit. The auxiliary feedback of Fig. 6 is similar to the auxiliary feedback of Fig. 4. The main feed-back of Fig. 6 between grid electrodes G3 and G2 is obtained by means of resistor R4 which serves to impress on the main. control electrode G3 a potential proportional to the current in the series tuned output circuit II. I This main feed-back path can be traced from one side of tuned circuit ll through the resistor R4, through batteries l3, l4 and I5 in series, and through resistor R5 back to the other side of tuned circuit I I. This type of circuit also produces current-controlled negative resistance and will sustain oscillations in circuit ll at the frequency at which maximum current is produced; i. e., the resonant frequency of this tuned circuit.

Fig. 7 shows a preferred form of voltage-controlled negative resistance oscillation generator in accordance with the invention, wherein there is employed an improved type of evacuated electron discharge device 16. This electron discharge device is known as a pentagrid vacuum tube and may be of the type sold commercially by the RCA Manufacturing Co., Inc., as the 2A7, or 6A7, or 6A8, and consists of a cathode K, five grids (G1, G2, Gs, Ge, GS) and an outer anode A, in the order named, as shown in the drawing. The tube differs from the previously described tubes used in Figs. 1 to 6, in that an additional screen grid Gs is provided between the main control electrode G3 and auxiliary anode A, and another screen grid G5 is provided between the main control electrode G3 and the main output anode G2. In the construction of the tube [6, output anode G2 may be placed to one side of the direct electron path from the cathode to the other electrodes. Main output electrode G2 of tube IE will therefore have a higher internal resistance than the main output electrode G; of tube I of Figs. 1 to 6. Similarly, auxiliary anode A of tube I6, because of the presence of the screen grid Gs, will have a higher internal resistance than auxiliary anode A of tube l of Figs. 1 to 6. The increase in internal resistances of the main anode G3 and auxiliary anode A increases the amplifying capabilities of the tube and therefore vention is not-limited to the precise arrangements described in the specification and shown in the drawing, since various modifications may be made without departing from the spirit and scope of the invention. For example, a vacuum tube of the-type shown in Fig. '7 can be used to replace any of the vacuum tubes shown in Figs. 3 to 6, merely by connecting the screen electrodes GS and Gs to a fixed positive potential with respect to the-cathode, as is done in Fig. '7. Al-

though the main anode electrode or main output electrode "has been shown in the various figures as a grid,it should be understood that in practice, this electrodeneed not be a grid but may consist of two small flat anodes placed to one side of the main electron stream. Other changes may readily suggest themselves to those skilled in the art, from a reading of this specification.

What is claimed is:

1. An oscillation generator comprising an electron discharge device having a cathode, a first grid, a second grid, a third grid, and another electrode, a feed-back circuit between said second and third grids for the production of negative resistance, and an auxiliary feed-back path from said other electrode to said first grid for augmenting the efiective negative transconductance produced between said second and third grids, and means for biasing said first grid negatively with respect to the cathode to prevent said first grid from drawing appreciable current.

2. The method of operating an electron discharge device utilizing the negative transconductance phenomena and having a cathode, an auxiliary control grid, a main anode, a main con- .trol grid, and an auxiliary anode, in the order named, which comprises supplying a control potential to said main control grid to vary the current on said main anode and said auxiliary anode, feeding-back energy from said main anode :to said main control grid to sustain oscillations, feeding back energy from said auxiliary anode to said auxiliary control grid to assist the control action of said main control grid, and maintaining said auxiliary control grid at a negative potential with respect to said cathode.

3. In combination, an electron discharge device comprising a cathode, a plate, and first, second and third electrodes spaced between said cathode and plate; means for applying positive potentials to said second electrode and to said plate relative to said cathode; means for applying a potential to said first electrode which is negative relative to said cathode; arsed-back circuit between said plate and first electrode including an impedance network located between said plate and cathode; a resonant circuit connected between said second electrode and cathode, and means coupling said third and second electrodes to cause said third electrode to swing in phase with said second electrode, whereby said potentials relative rto 1 said cathode; .a feedeback pathrbetweenxsaid main output electrode and said main -control grid; aiparallel tuned circuit between said main :output electrode. and cathode; and .:an auxiliary; feed-back path between said auxiliary anode and said. auxiliary control grid.

5. An oscillation: generator comprising an electron-discharge device having a cathode, an auxiliaryicontrol grid, a main anode, a main control grid; and". auxiliary anode in the order named; means:for-biaising'said auxiliary control grid negatively relative to said cathode; and means for maintaining said main anode and said auxiliary anode at positive potentials relative to said cathode; a feed-back path between said main anode and 'saidmain'control grid, including a battery forv maintaining said main control grid negative relative to said main anode; a parallel tuned circuit between main anode and cathode; and an auxiliary feed-back path between said auxiliary anode and said auxiliary control grid.

6. An oscillation generator comprising an electron discharge device having a cathode, an auxiliary control grid, a main anode, a main control grid, and an auxiliary anode in the order named; means for biasing said auxiliary control grid negatively relative to said cathode; and means for maintaining said main anode and said auxiliary" anode at positive potentials relative to said cathode; a feed-back path between said main anode and said main control grid, said feed-back path including a series tuned circuit of inductanceand capacitance; and an auxiliary feedback path between said auxiliary anode and said auxiliary control grid.

7. An oscillation generator comprising an electron discharge device having a cathode, an auxiliary control grid, a main anode, a main control grid, and an auxiliary anode, in the order named; means for biasing said auxiliary control grid negatively relative to said cathode; and means for maintaining said main anode and said auxiliary anode at positive potentials relative to said cathode; a feed-back path between said main anode and said main control grid, said feed-back path including a series tuned circuit of inductance and capacitance; and an auxiliary feed-back path between said auxiliary anode and said auxiliary control grid, said auxiliary path including a condenser directly connected between'said auxiliary anode and said auxiliary control grid.

8. An oscillation'generator comprising an electron discharge device having a cathode, an auxiliary control grid, a main anode, a main control grid, and an auxiliary anode, in the order named; means for biasing said auxiliary control grid negatively relative to said cathode; and means for maintaining said main anode and said auxiliary anode at positive potentials relative to said cathode; a capacitive feed-back path between said main anode and said main control grid; a

' parallel tuned circuit between said main anode and cathode; and an auxiliary feed-back path between said auxiliary anode and said auxiliary control grid.

9. An oscillation generator comprising an electron discharge device having a cathode, an auxiliary control grid, a main anode, a main control grid, and an auxiliary anode, in the order named; means for biasing said auxiliary control grid negatively relative to said cathode; and means for maintaining said main anode and said auxiliary anode at positive potentials relative to'said cathode; a feed-back path between said main anode and said main control grid including a potential source for maintaining said main control grid at a negative potential relative to said main anode, said feed-back path also including a series tuned circuit; and an auxiliary feed-back path between said auxiliary anode and said auxiliary control grid.

10. An oscillation generator comprising an electron discharge device having a cathode, first, second and third electrodes, and another electrode, in the order named; a feed-back circuit between said second and third electrodes for the production of negative resistance; and an auxiliary feed-back path from said other electrode to said first electrode for augmenting the effective negative transconductance produced between said second and third electrodes; and means for biasing said first electrode negatively with respect to said cathode to prevent said first electrode from drawing appreciable current.

11. An oscillation generator comprising an electron discharge device having a cathode, an auxiliary control grid, a main output electrode, a main control grid, and an auxiliary anode in the order named; means for biasing said auxiliary control grid negatively relative to said cathode; and means for maintaining said main output electrode and said auxiliary anode at positive potentials relative to said cathode; a feedback path between said main output electrode and said main control grid for the production of negative resistance; means for the utilization of said negative resistance for the maintenance of oscillations; and an auxiliary feed-back path between said auxiliary anode and said auxiliary control grid.

12. An oscillation generator in accordance with claim 8, including additional grid electrodes for screening said main control grid from both said auxiliary anode and from said auxiliary control grid, and a connection from said additional grid electrodes to said cathode.

13. An oscillation generator in accordance with claim 8, including additional grid electrodes for screening said main control grid both from said main anode and from said auxiliary anode, and a connection including a source of polarizing potential from said additional grid electrodes to said cathode.

14. The method of operating an electron discharge device utilizing the negative transconduc-tance phenomena and having a cathode, an auxiliary control grid, 2. main anode, a main control grid, and an auxiliary anode, in the order named, which comprises feeding back energy from said main anode to said main control grid to sustain oscillations, feeding back energy from said auxiliary anode to said auxiliary control grid to assist the control action of said main control grid, and maintaining said auxiliary control grid at a negative potential with respect to said cathode.

EDWARD W. HEROLD. 

